In recent years, mankind has realized that elemental mercury is a serious environmental hazard. It has therefore become of increasing concern to many industrial facilities to remove mercury from its waste byproducts, before disposing of them.
Levels of mercury in the concentrate fed to the zinc roasters at the Cominco Ltd. plant in Trail, British Columbia, have been increasing over time due to an increasingly higher mercury content from ore concentrates of the Red Dog and Sullivan mines. This level of mercury results in more than 20 tonnes per year of crude calomel being formed. Since the crude calomel has no market and cannot be disposed of without environmental danger, the continual production of crude calomel leads to a storage space problem. Options for disposing of the crude calomel include building long term storage facilities, or converting the calomel to metallic mercury or upgrading the crude calomel to a marketable calomel that meets American Chemical Society (ACS) standards.
Over the years, a number of processes have been developed to refine mercury or remove it from various mediums.
U.S. Pat. No. 4,640,751, granted Feb. 3, 1987, Dyvik et al., assigned to Boliden AG., discloses a method for the purification of gases containing mercury and simultaneous recovery of the mercury in metallic form by a process where primarily a reaction takes place between metallic mercury vapour and mercury (II)-chloride compounds in solution in a liquid phase. During this process, the formation and deposition of only slightly soluble Hg (I)-chloride (calomel) occurs. The deposited calomel is oxidized to easily soluble Hg (II)-chloride compounds by the addition of chlorine. At least some of the chlorine used is recovered by electrolysis of the formed Hg(II)-chloride.
U.S. Pat. No. 5,013,358, granted May 7, 1991, Donald L. Ball et al., assigned to Cominco Ltd., discloses a method for the recovery of mercury from mercury-containing material. Insoluble mercury salts and any mercury in mercury-containing material are converted into a soluble form by controlled chlorination. The soluble forms of mercury in the chlorination solution are reduced with iron, preferably iron powder, to elemental mercury. After separation from the reduced solution, the solids from the reduction containing entrained mercury, are subjected to a separation step, which separates and substantially quantitatively recovers substantially pure mercury. Separation by elutriation through a body of mercury is preferred. Prior to separation, the reduction solids may be kneaded for coalescence of fine mercury particles, followed by slurrying of the kneaded material. Any selenium in the reduced solution may be recovered in a reduction with a suitable reductant, preferably by adding strong sulfuric acid in the presence of the ferrous chloride formed in the preceding reduction, and excess sulfur dioxide. The process is carried out at ambient conditions, and the amount of liquid in the process is controlled. Substantially no mercury is discharged from the process in residues, or residual liquid. The shortcoming of this process is that elemental mercury is recovered. Elemental mercury is now difficult to sell or to dispose of because of the environmental hazard.
U.S. Pat. No. 4,729,882, granted Mar. 8, 1988, assignee Tokyo-to Kankyo Seibi, discloses a process whereby waste gas containing mercury and a substance containing chlorine are heated and the mercury is changed to soluble mercury chloride (HgCl.sub.2). The produced mercury chloride is washed with a washing liquid to fix the mercury as a stable chlorocomplex ion (HgCl.sub.4(-2)).
U.S. Pat. No. 5,071,475, granted Dec. 10, 1991, Barreau et al., discloses a process and installation for producing mercury by reduction of calomel by implementing a process for preparing metallic mercury. The installation essentially comprises a reaction vessel with an inclined base for the reduction provided with an agitator. The vessel is connected by a conduit to a decanter and provided with water supply means and sulphuric acid supply means. A mercury recovery tank is connected to the lower part of the reaction vessel.
U.S. Pat. No. 3,849,267, granted Nov. 19, 1974, Hilgen et al., discloses a process for recovering mercury from a mercury-containing gas which includes mixing chlorine with the gas. The mixture is then passed through a gas-permeable bed of a non-porous solid material which has a large surface area in relation to the bed volume to collect necessary chloride thereon. The mercury is thereafter recovered by either washing the bed with chlorine containing brine and passing the resulting mercury-containing brine to an electrolysis cell with a mercury cathode, or dissolving the bed material in an aqueous process stream and passing the stream to an electrolysis cell with a mercury cathode.
The following patents disclose subject matter which is of general interest to the area of mercury and calomel technology.
U.S. Pat. No. 740,855, granted Oct. 6, 1903, Von Hoessle, discloses a colloidal calomel product having a dull-white to grey powder quality. The powder contains mercurous chloride (Hg.sub.2 Cl.sub.2) in a water-soluble form. The product is easily soluble in water to an opalescent liquid and capable of being precipitated from aqueous solution by addition of acid. This patent does not disclose purification of calomel.
U.S. Pat. No. 1,809,449, granted Jun. 9, 1931, Lindsay, describes a white silvery variety of mercurous chloride obtained as a precipitate by reduction of mercuric chloride, in aqueous solution, and in the presence of hydrochloric acid, with an aqueous solution of stannous chloride. This patent also does not describe purification of calomel.
U.S. Pat. No. 2,570,408, Van Gorder, granted Oct. 9, 1951, discloses a process for producing mercurous chloride crystals having a silky nacreous lustre which comprises reacting mercuric chloride with a soluble inorganic dibasic phosphite at a temperature from about 10.degree. to 95.degree. C. in a solution containing from about 1.5 to 7% by weight of the mercuric chloride, soluble, inorganic phosphite and a chloride such as sodium or potassium chloride. The reaction mixture is acidified and the mercurous chloride crystals formed are washed without material loss of lustre. This technology pertains to synthetic pearls.
U.S. Pat. No. 3,704,103, Barta, granted Nov. 28, 1972, discloses a method of preparing single crystals of mercurous halides, which comprises separating the respective mercurous halide from the ambient atmosphere, heating the system to a temperature of at least 120.degree. C. and cooling to the respective crystallization temperature after a specified pressure has been reached. The mercurous halide is gradually cooled to a temperature wherein mercurous halide crystallizes. This patent does not pertain to calomel.
U.S.S.R. Patent SU1567520A1, granted May 30, 1990, Emelyanova, discloses a method for increasing yield and purity of mercury chloride. Metallic mercury is treated by a water solution of hydrogen chloride while passing ozone through the solution. By using ozone, the yield of product is above 99%.